1. Technical Field
The present invention relates, generally, to asymmetrical digital subscriber line (ADSL) systems and, more particularly, to an improved hybrid circuit for use in connection with ADSL data communication.
2. Background Information
Due in part to the widespread popularity of the Internet, consumers and Internet Service Providers (ISPs) continue to seek improved, higher-bandwidth methods of providing data communication. In this regard, a great amount of interest has been generated by Asymmetric Digital Subscriber Line (ADSL) systems. In general, ADSL technology involves the use of modem systems configured to communicate over a conventional twisted-pair copper line (often referred to as the “last mile”) to provide, in addition to traditional voice telephone service, approximately 1.5 to 8.0 Mbps data transmission downstream (i.e., to the subscriber), and about 16 to 640 Kbps data transmission upstream (i.e., to an Internet service provider (ISP) or other data network).
ADSL is one of a family of acronyms based on digital subscriber lines (DSL), including, for example, SDSL (symmetric digital subscriber line), RADSL (rate adaptive ADSL), HDSL (high data rate digital subscriber line), and VDSL (very high data rate digital subscriber line). This DSL class of technology is sometimes generally referred to “xDSL.” Thus, while the acronym “ADSL” will be used throughout this application, it should be appreciated that this is not intended as a limitation on potential applications.
Currently known ADSL systems are unsatisfactory in a number of respects. For example, conventional ADSL transceivers include a hybrid circuit which essentially functions as an electrical bridge whose impedance is configured to match, as close as possible, the telephone loop impedance. However, as the impedance of the telephone loop is not precisely fixed, it is impossible to design the bridge such that its impedance matches all possible environments in which it is likely to be deployed. As a result of this impedance mismatch, there is always a significant echo return loss through the hybrid circuit. For example, it is not unusual for conventional hybrid circuits to experience an echo return loss of between about 20 dB and 40 dB.
Furthermore, currently known hybrid circuits are unsatisfactory in that they necessarily incorporate a high-pass filter designed to implement frequency-division multiplexing of the upstream and downstream signals. This filter, which may be a 5th order or 7th order filter, includes a large number of components and therefore greatly increases the size and cost of the hybrid circuit. Similarly, such circuits require a large magnetic transformer to provide isolation of the telephone line from the analog front end. In the context of the central office, any circuitry that increases the size of the hybrid is disadvantageous in that space within the central office is a premium.
Methods are therefore needed in order to overcome these and other limitations of the prior art.